The present invention is related to the field of fibre optic communications and networks, and more particularly, provides tuned optical filters, filter systems, and methods for filtering and tuning of optical filters which are especially useful in dense wavelength-division multiplex systems.
A variety of optical filters have been developed to differentiate optical signals based on their wavelength. For example, thin film optical filters can selectively pass signals having wavelengths that are less than a nominal maximum wavelength. Such low wavelength pass filters can be combined with similar thin film filter structures which selectively pass optical signals having a wavelength greater than some minimum nominal wavelength. By careful selection of the individual filter structures and the corresponding minimum and maximum nominal wavelengths, multicavity band pass filters having fairly narrow wavelength transmission ranges can be produced.
Unfortunately, a single narrow band pass filter can not always provide the desired filtering performance for dense wavelength division multiplex systems. Specifically, some portion of the optical signal which is outside of the nominal range of a multicavity narrow band pass filter typically "leaks through" with the filtered signal. This leakage can lead to cross talk between signals of differing wavelengths.
Work in connection with the present invention has shown that it can be helpful to isolate the discrete individual wavelengths in a multiplex system using two or more multicavity narrow band pass filters in series. For such multifilter arrangements to substantially enhance filtering performance, the two filters should operate at wave-lengths which are as identical as possible. In particular, the central wavelength value of narrow band pass filters used in series should be substantially the same, or each filter will operate individual at the upper or lower ends of the nominal range, and filter performance will not be improved (beyond that of a single filter structure) to the full extent possible.
Unfortunately manufacturing limitations for narrow band pass filters limit the number of filters having identical central wave-lengths. Hence, the usable filter pairs in each coating run is quite limited. This limited yield greatly increases the cost of matching or "tuned" narrow band pass filters. Work in connection with the present invention has improved matched filter yield somewhat by selectively baking filters in a particular temperature sequence to shift the filter central wavelength using the elastic theory of material. Although such selective baking can shift the filter wavelengths, the accuracy of this method is somewhat limited, so that several bakings are often needed. As baking is time consuming and can degrade the optical characteristics of the filters, some alternative mechanism for varying the central wavelength of narrow band pass filters is desirable.
The present invention solves or substantially mitigates the above problems by providing a mechanism for mechanically tuning optical filters with very high degrees of accuracy and repeatability, and allows the tuning adjustment range and sensitivity to be tailored for a particular filtering application.